Many terpolymers based on TA and HMD described in the patent literature or in commercial existence give very marginal performance in these applications. Such is the case for 6T/DT [50/50 molar ratio], the copolymer of hexamethylene terephthalamide (6T) and 2-methyl pentamethylene terephthalamide (DT) (U.S. Pat. No. 5,378,800 to Mok, Pagilagan; U.S. Pat. No. 5,322,923 to Lahary, Coquard) because of its high glass transition temperature (Tg) and high post-molding shrinkage. A terpolymer of hexamethylene terephthalamide/hexamethylene isophthalamide/hexamethylene adipamide (6T/6I/66) terpolymer [65/25/10 molar ratio], based on 3 acids, TA, isophthalic acid (IA) and adipic acid (AA), and HMD (U.S. Pat. No. Re. 34,447 to Poppe, Lisle, et al.) has high mold and post molding shrinkages when molded in cold molds. Similar would be the situation for the hexamethylene terephthalamide/hexamethylene isophthalamide/hexamethylene sebacamide (6T/6I/610) terpolymer [70/20/10 molar ratio] (U.S. Pat. No. 5,424, 104 to Amimoto et al.) because of its high Tg. Such would also be the case of the copolymer of hexamethylene terephthalamide/caprolactam (6T/6) [70/30] (EP 413258, to Kopietz, Betz, Blinne, Koch) because of its high T.sub.g. Copolymers of hexamethylene terephthalamide/hexamethylene adipamide (6T/66), based on TA, AA and HMD, at 55/45 molar ratio (U.S. Pat. No. 5,424,104) or 65/35 molar ratio (U.S. Pat. No. Re. 34,447) are functional in these "cold mold" injection molded parts, and display acceptable post mold shrinkage, but suffer in melt processability (i.e. thermal stability and flow) and moisture adsorption.
Terpolymers containing TA and HMD are generically described in the patent literature. Typical of that prior art is U.S. Pat. No. 5,200,450 to Blinne, Goetz, Hurley, Gutsche, Muehlbach (BASF) wherein the semi-aromatic polyamide is composed of i) 20-90% (wt) of units derived from terephthalic acid and hexamethylene diamine, ii) 0-50% (wt) of units derived from .epsilon.-caprolactam, iii) 0-80% (wt) units derived from adipic acid and hexamethylene diamine, and iv) 0-40% (wt) of further polyamide-forming monomers. Also typical of that prior art is U.S. Pat. No. 5,324,766 to Ikejiri, Yamamoto, Kawamoto (Mitsui Petrochemicals Company), wherein the semi-aromatic polyamide resin consists essentially of i) a dicarboxylic acid component recurring unit consisting of 30-100 mole % of terephthalic acid component unit and 0-40 mole % of an aromatic dicarboxylic acid component unit other than terephthalic acid and/or 0-70 mole % on an aliphatic dicarboxylic acid component unit having an alkylene group of 4-20 carbon atoms and ii) a diamine component recurring unit comprising an alkylene diamine component unit having an alkylene group of 4 to 25 carbon atoms.